a ~4GB MP2 Movie file -> 3ivx

Yes, it takes a long time to open a big MPEG-2 video, just wait longer.

MPEG-4 can be both a file type and a video codec, which is why it's in the first popup menu when exporting, but 3ivx is just an encoder, and not a file type. 3ivx should show up if you export to QuickTime Movie and then go into Options, Video - Settings.

3ivx is now a little more official than it used to be, there are various products that you can buy. I have one of those products installed, which may be why I get 3ivx at the top of the Compression Type menu.

The 3ivx page now talks about an evaluation copy. It may take some reading to figure what is limited about that version. Start at this page and make sure you're up to date and use their latest installer:

http://www.3ivx.com/download/index.html

{edited}. I just bought the mpeg-2 componet from apple for 20 dollars to just be able to open it. Now i have to pay another 20 to save it!

Am a little confused by both what you want to do and how you are trying to do it. If the source movie is MPEG-2 and if it has "muxed" audio/video data, then you really don't want to use QT to convert it in the first place since you will lose the audio content. However, as you already have the QT MPEG-2 component and 3ivX (assume it is the free D4 v4.5.1 here) components installed, one possible work flow would be MPEG Streamclip (free). With respect to the 3ivX component, it does not show up in the base export pop-up window, but is found as one of the video settings options under the "Movie to QT Movie" pop-up option in either QT Pro, iMovie, FCE, FCP, or the "Movie to QT Movie" menu option in MPEG Streamclip.

Not helpful... I already know that 3ivx should be in the menu... But it isn't ... It just ain't . And i've installed it 2 times.

As indicated above, my answer was predicated on 3ivX D4 v4.5.1 which I believe pre-dates core-duo technology. Since you have not indicated whether or not you have sought out and located a universal binary version of this software, I would assume you may have a basic compatibility issue here. Have you tried running under Rosetta? Is your source file a "muxed" MPEG-2 format?